The present invention relates generally to integrated circuit (IC) designs, and more particularly to a high voltage tolerant input buffer that is operable in under-drive conditions.
An IC includes a number of input pads, which may be used for receiving external signals. Each input pad is typically connected to a corresponding input buffer. An incoming signal must pass through the input buffer before entering core circuitries within the IC.
A conventional high voltage tolerant input buffer typically includes an NMOS pass-gate transistor, an input regulating module, a level down module, and some kind of electrostatic discharge (ESD) protection module. The NMOS pass-gate transistor is implemented to protect the input regulating module when the voltage at an input pad becomes too high. Since the voltage level of the output signals generated by the input regulating module is often higher than the voltage level at which the core circuitries operate, the level down module is implemented to convert the output signals from a higher voltage level to a lower voltage level before passing them on to the core circuitries.
However, the conventional high voltage tolerant input buffer may fail in under-drive conditions where the supply voltage of the buffer drops below a predetermined voltage level (e.g. 1.8 volts). When such under-drive condition occurs, the MOS transistors within the input regulating module may not be fully turned on as they were supposed to be. As such, the input regulating module may not operate properly.
Therefore, it is desirable to have a high voltage tolerant input buffer that can operate properly in under-drive conditions.